Normal animal fibroblasts cultured in vitro require the presence of blood serum factors for growth. We have purified to homogeneity a growth factor polypeptide from human serum that stimulates DNA synthesis and cell proliferation of mouse and human fibroblasts (PNAS 72:2603, 1975). Biological assays with cell culture and a specific radioimmunoassay for the human serum derived growth factor (PNAS 74:1973,1978) have demonstrated that the human serum growth factor polypeptide is largely derived from platelets. This platelet derived growth factor (PDGF), stored in platelet alpha-granules, is released into serum during clotting. PDGF has been purified to homogeneity by a process which includes heat (100 degrees) treatment of washed platelets, followed by ion-exchange chromatography, gel filtration in l M acetic acid, isoelectric focusing and preparative SDS polyacrylamide gel electrophoresis. PDGF has a PI of about 9.8 and a molecular weight of about 13,000 on reduced SDS-PAGE gels. These properties of PDGF are similar to those of the human serum growth factor previously isolated in our laboratory (PNAS 72:2603, 1975). The specific activity of the purified PDGF is 20 million times greater than that found in unfractionated human serum. Purified PDGF stimulated replicative DNA synthesis and cell proliferation at 10 to the minus 10th power M; about 10 to the sixth power molecules per cell stimulate a round of DNA synthesis. In cell culture, PDGF induces the initial event in the replicative response "competence". Platelet-poor plasma (PPP) induces a later event, "progression," which allows competent cells to pass through G0/G1 and enter the S phase. These two sets of factors, in platelets and PPP, act synergistically to stimulate growth of mammalian cells in culture.